Electrocardiograms (ECGs) are graphic depictions of electrical activity in the heart. ECGs are produced by electrocardiographs which are available as stand alone devices, portable devices, and are also integrated into various types of vital signs recording and monitoring devices. ECGs are depicted by time (ms) versus voltage (μV) and typically are represented by five or six points as a waveform. The typical five data points on an ECG, as shown in FIG. 1A, are the P wave, QRS complex (represented as the combination of the Q, R, and S waves respectively), and T wave. The less frequently seen sixth point is a U wave. The data produced from the graphical depictions are useful in diagnosis of patients to determine what, if any, and the extent to which heart-related problems exist in a patient. For instance, ECGs are used in diagnosing: cardiac arrhythmias (irregular heart rhythms), myocardial infarction (heart attacks), hyper- and hypokalemia (high or low potassium levels, respectively), blockage, ischemia (loss of oxygen due to lack of blood flow possibly from blockage), and may also assist in diagnosis of non-heart related ailments. Accordingly, ECGs are known and proven to be valuable tools in diagnosis heart and even non-heart-related problems with patients.
Particularly, the ECG waveforms are useful in determining whether certain conditions exist or the predisposition of such conditions occurring based on established patterns. As shown in FIG. 1A, ECGs are defined by several characteristics in the PQRST waveform. Particularly, important information can be derived by measuring the time between certain waveforms; commonly reviewed time intervals are those between the P wave and the beginning of the QRS interval (known as the PR interval) and the time between the QRS complex and the T wave (known as the QT interval), which are shown in FIG. 1A. Further, as shown in FIG. 1A, there are other relevant data from the PR segment, the QRS complex, and the ST segment.
Typically, ECGs are used as diagnostic tools in various settings such as hospitals and doctors offices. ECGs taken in such instances are generally limited in scope, to the time of minutes, and therefore may not always provide sufficient information for diagnosis or data for analysis. Accordingly, there are known in the art portable devices used for recording ECGs and other patient data for extended periods of time where required to assist in diagnosis, monitoring, or other analysis measures.
Portable electrocardiography devices are known devices in the art for use in recording ECGs on ambulatory patients. These devices are also known as Holter monitors. The devices have significance in the medical industry as they can record ECGs for longer periods of time, such as minutes, hours, or days, and also in intervals of the same sort, as prescribed by a physician, without restricting the patient to doctor's offices or a hospital bed. The purpose of having prolonged measurements of such devices is useful to assist in diagnosing whether heart conditions are present in a patient, and upon individual review of the actual data, what types of conditions may exist. Further, such devices can assist simply for monitoring the heart in situations where the patient may be at risk for certain conditions, which can be identified from patterns displayed on the ECG, but which may also not be readily apparent from the few minutes of data typically gathered in a doctor's office.
Because of their physical size, portable devices used to have less robust technical specifications (i.e. limited number of recorded channels, lower sampling rates, and smaller voltage resolution). However, most recent models are now matching the technical specification of standard resting ECG machines. Currently, several commercial companies offer today high resolution Holter monitors with 12-lead storage at 1000 Hz sampling rate with a few microvolts resolution (e.g. H12+ recorder from Mortara Instruments, the CM3000 recorder from Getemed-GE, or the Spiderview recorder from ELAMedical).
As mentioned above, Holter monitors can be used for monitoring patients for various purposes. One such purpose can be monitoring patients undergoing clinical trials for various procedures or drug testing, specifically in FDA clinical trials for approval of devices and drugs onto the market. Emerging FDA and global regulatory guidelines require all new drug candidates to undergo more rigorous Phase I QT studies conducted by a centralized laboratory using validated, digital analysis of ECG waveforms and manual QT interval determinations by designated cardiologists; this type of study is commonly referred to as the Thorough QT study or TQT study. As an example, electrocardiography data is continuously recorded up to 72 hours to monitor ischemia, ventricular and supra-ventricular dysrhythmias, conduction abnormalities, QT interval, and heart rate variability.
Further, certain clinical studies require that ECGs be taken when the concentration of a drug in a patient's circulatory system is at a predetermined level. It is often important to take the ECG when the level of drug is at its maximum to determine the possibility of certain conditions on the heart by such experimental drugs.
Phase I studies may last as long as two weeks. During that time test subjects wear Holter monitors which record ECG data while letting the subject remain ambulatory. This is often a requirement since test subjects do not wish to remain in bed for the duration of a Phase I trial.
While portable devices are known in the art for ambulatory patients, there are several issues that arise during the use of these devices. For instance, while data is recorded, the signal can either receive interference or pick up “noise” which is reflected on the ECG. Another important factor that needs to be accounted for in ambulatory patients, particularly for QT studies, is that the ECG measurements should be taken when the heart rate is stable. In other words, the ECG measurements should be relatively consistent or has minimal or no variation from one ECG measurement to the next. Indeed, many parameters such as the QT interval need some time, generally a few minutes, to reach a steady state after occurrence of a heart rate variation, such as heart rate acceleration or deceleration. This phenomenon is known in the literature and it is commonly referred to as the hysteresis heart rate effect. Ideally relevant measurements should thus be taken in a clean and hysteresis-free segment of the ECG signal.
As mentioned above, problems related to noise, interference, varied heart rates are known. In the case of Holter monitors, the patient is moving with 12-leads attached to his or her body. Such motion can and does generate noise in the sensed ECG signals. In addition, ECG signals are not reliable when the patient's heart is accelerating or decelerating. Other issues can result as such devices are susceptible to electromagnetic interference from power lines and other environmental factors. Thus, prior art devices and methods do not account for these dynamic condition which interfere with reliable readings.
Therefore, a method of extracting data free from noise and from segments of stable heart rates from recorded ECG data, collected from Holter and similar monitors, is desired.
Further, an apparatus that can utilize a method for collecting optimum ECG data is desired.